Conventionally, metallurgical slag is granulated in water.
Water quenching ensures fast solidification of the metallurgical slag, which, in the case of blast furnace slag, is a necessary condition for obtaining a valuable product. A water jet is firstly used to fragmentize the hot liquid slag stream into very small particles and to transfer them into a water bath. The energy form the hot slag is withdrawn through direct contact between the hot liquid slag and the water. As this has to happen at ambient pressure, the temperature of the slag is immediately lowered to a temperature level of below 100° C.
During the water granulation process, the sulfur contained in the hot liquid slag reacts with the water and generates sulfur dioxide (SO2) and hydrogen sulfide (H2S). The quantity of these toxic and malodorous gases is mainly dependent on the chemical composition of the slag and the granulation parameters.
The concentrations of both sulfur dioxide and hydrogen sulfide may be too high with regard to the environmental legislation so that measures have to be taken to lower these emissions to an acceptable concentration.
In the state of the art (see e.g. EPO 573 769, LU 88 441 and/or LU 91 424, from Paul Wurth S.A.) a condensation tower is erected above the granulation area. In the condensation tower, both SO2 and H2S are condensed with the water vapor and hence recombined with the water to form sulfuric acid (H2SO4). The water is pumped into a cooling tower in which, due to contact with the ambient air under turbulent conditions, sulfur is partially transferred from the hot process water to the cooling air. Due to the large amount of cooling air, however, only small concentrations of SO2 and H2S (below 1 mg/Nm3) are measured.
JP2002308655 discloses a method to mix hot liquid slag with iron-containing fine powder to adjust and control the density of the slag. The addition of oxidized iron powder decreases the nitrogenated particles trapped in the molten slag and thus allows to obtain a high density slag independent of the tapping temperature of the slag.
CN 101 545 018A describes a process and a device wherein the ladle slag first enters the drum slag area from the funnel. There are steel balls in the slag area, and as the drum turns, the slag is cooled by the steel balls. The heat of the molten steel and the slag is quickly removed by the steel balls, and the slag is plasticized. After solidification, it is carried by the rolling steel balls into the water cooling area. The slag and the steel balls are simultaneously cooled by the water. Then it is carried to the outer drum body through the drum's lattice-like grate bars, and the slag is cooled once again by the water in the outer drum and finally discharged. During this process, due to the effect of the fast rolling of the steel balls, the slag and steel are separated. The amount of cooling water is 1.25 T to 2 T per ton of slag. The steel ball rolling speed is 0.5 m/s to 2 m/s. The speed of the rolling balls makes sure the steel balls are not enveloped in the cooling molten steel. A suitable steel ball rolling speed guarantees that the granulation of the slag and the separation of the molten steel. It prevents lumping in the cooling process.